1. Field of the Invention
The present invention relates to the field of cell culture medium formulations, and more specifically, to methods for continuously preparing cell culture medium formulations and buffered salt solutions from selected subgroups of medium concentrates.
2. Related Art
Cell culture medium formulation provide nutrients necessary to maintain and grow cells in a controlled, artificial and in vitro environment. Characteristics and compositions of the cell culture mediums vary depending on the particular cellular requirements. Important parameters include osmolarity, pH, and nutrient formulations.
Medium formulations have been used to grow a number of cell types including animal, plant and bacterial cells. Cells grown in culture medium catabolize available nutrients and produce useful biological substances such as monoclonal antibodies, hormones, growth factors and the like. Such products have therapeutic applications and, with the advent of recombinant DNA technology, cells can be engineered to produce large quantities of these products. Thus, the ability to grow cells in vitro is not only important for the study of cell physiology, it is necessary for the production of useful substances which may not otherwise be obtained by cost-effective means.
Cell culture medium formulations have been well documented in the literature and a number of medium are commercially available. Typical nutrients in cell culture medium formulations include amino acids, salts, vitamins, trace metals, sugars, lipids and nucleic acids. Often, particularly in complex medium formulations, stability problems result in toxic products and/or lower effective concentrations of required nutrients, thereby limiting the functional life-span of the culture medium. For instance, glutamine is a constituent of almost all medium formulations that are used in the culturing of mammalian cells in vitro. Glutamine decomposes spontaneously into pyrrolidone carboxylic acid and ammonia. The rate of degradation can be influenced by pH and ionic conditions but in cell culture medium, formation of these breakdown products cannot be avoided (Tritsch et al., Exp. Cell Research, 28:360-364(1962)).
Wang et al. (In Vitro, 14:(8):715-722 (1978)) have shown that photoproducts such as hydrogen peroxide, which are lethal to cells, are produced in Dulbecco's Modified Eagle's Medium (DMEM). Riboflavin and tryptophan or tyrosine are components necessary for formation of hydrogen peroxide during light exposure. Because most mammalian culture medium formulations contain riboflavin, tyrosine and tryptophan, toxic photoproducts are likely produced in most cell culture mediums.
To avoid these problems, researchers make medium formulations on an "as needed" basis, and avoid long term storage of the culture medium. Commercially available medium formulations, typically in dry powder form, serve as a convenient alternative to making the medium formulations from scratch, i.e., adding each nutrient individually, and also avoids some of the stability problems associated with liquid medium formulations. However, only a limited number of commercial culture medium formulations are available, except for those custom formulations supplied by the manufacturer.
Although dry powder medium formulations may increase the shelf-life of some medium formulations, there are a number of problems associated with dry powdered medium formulations, especially in large scale application. Production of large volumes requires storage facilities for the dry powder, not to mention the specialized kitchens necessary to mix and weigh the nutrient components. Due to the corrosive nature of dry powder medium ingredients, mixing tanks must be periodically replaced.
There exists a need to lower the cost of production of biological substances. Efficient and cost effective methods to stabilize liquid cell culture medium formulations as well as the development of convenient methods to produce 1.times.medium formulations would be an important development in the field of cell culture medium technology.
One such development in the field of cell culture medium formulations is the development of liquid medium concentrates as is disclosed in U.S. Pat. No. 5,474,931 issued to DiSorbo et al. on Dec. 12, 1995 ("DiSorbo"). DiSorbo discloses a method of subgrouping medium formulations into stable, compatible components that can be solubilized at high concentrations (10.times. to 100.times.). Concentrated culture medium formulations (2-10.times.) or 1.times.cell culture medium formulations can be prepared by mixing a sufficient amount of the concentrated subgroup solutions with each other and with a sufficient amount of a diluent (water, buffer, etc.).
Escalating demand for large volumes of nutrient medium and buffered salt solutions and increasing pressure to minimize batch-associated costs, such as sterile filtration and quality release testing, has driven a requirement for increased production batch sizes of liquid medium. As a result, stainless steel formulation tanks of 5000-10,000 liters for preparation of large batches of liquid medium or buffered salt solutions have become relatively common. However, scale-up manufacture of these fluids in this manner presents challenges regarding product quality and economy.
What is needed is a system and method for providing continuous, online preparation of large volumes of biological fluids (e.g., liquid medium, buffered salt solutions, etc.) within a highly controlled manufacturing system.